Interference reduction system



Feb. 13, 1940. v Q LANDQN I 2,190,292

INTERFERENCE REDUCTION SYSTEM Filed July 30, 1938 OUTPUT LIN/TEE Zmoentor Vern n D. Landon 3g I attorney 55 pulses are applied to Patented Feb. 13, 1940 2,190,292 1 INTERFERENCE-REDUCTIONSYSTEM Vernon D. Landon; Haddonfield, N. "J; assig l or to Radio Corporation of of Delaware This invention relates to interference reduction systems such as are utilized to eliminateundesired electrical impulses from the output of radio receiverv apparatus or the like, and hasfor. its 5 principal object the provision of an improved system and method of operation whereby the output current of such apparatusis caused to assume a no-signal value when the level of the received'impulses exceeds a predetermined value.

In my copending application Serial No. 222,141,

filed July 30, 1938, assigned to the same assignee as the present application, there is disclosed a rejector limiter system in which the limiter cathode is swung positive with respect to the 15 anode potential and the anode current is minimized or interrupted when the level of the received impulses increases to a predetermined value. The system of the aforesaid application has great utility as a radio frequency limiter, 20 but is less satisfactory as an audio frequency limiter, for the reason that in the case of audio frequencies most effective elimination of the noise requires that anode current fiow'when the received impulse level is excessively high or during excessive positive .swings of the input potential.

In its more specific aspects, the present invention provides an audio frequency limiter wherein the output current of the limiter isnot inter- .rupted but automatically assumes a zero signal value when the received impulse level increases to or exceeds a predetermined value. To this end, there is interposed between the audio .frequency detector and the limiter an electron discharge device, the anode or plate current of which is interrupted in response, to a predetermined received impulse level, the output of the audio frequency limiter during this interruption period being maintained at the level it normally assumes when no signal is received.

The invention will be better understood from the following description, considered in connection with the accompanying drawing, and. its

scope is indicated by the appended claims. 45 Referring to thedrawing:

Figure 1 'is a wiring diagram of the improved limiter system,

1 Figure 2 is the control grid potential-plate current characteristic of the limiter device, and Figure 3 is the radio frequency input-limiter output characteristic of the improved system.

The system of Fig. 1 includes a tuned input circuit including a capacitor l0 and an inductor ll through which the modulated carrier imthe cathode and anode jbleeder resistor 2|, vice-l8. v

J facilitated. i

- acteristic; illustrated by Fig. 3 results. stated, the limiter output assumes the normal no-signal value when .the input becomes exces- 'SlVe. -jection period is highly advantageous in that the noise otherwise resulting from abrupt changes in output is avoided. 1

America, a corporation Application July so, 1938, Serial No. 222,142 8'Claims. (o1. 2511-20) electrodes of adetector l2. This input circuit also includes a radio frequency filter network I3-l ll5 and a resistor Hi. The detected modulation impulses produced in the resistor l6 are applied to thecontrol grid of an electron 5 discharge device l8 through M an input circuit which includes the grid H, the resistor IS, the groundterminal IS, the lower section 20 of a and the cathode of the de- M The cathode-anode or output circuit of the device l8 includes .a resistor 22 and asection 23 of the resistor 2|. With these connections, the

received impulses are amplified by the device l8 and applied to the control grid 24 ofthe limiter 25. 1

Thelimiter 25 is provided with an anode 26 to "which is applied a relatively low potential, a screen grid 2] to whicha relatively'high poten'-- .tial is appliedfrom the upper terminal of the resistor'section 28, ;a cathode 29 which is connected toground through a resistor 30, anda M ,resistor 3| which is connected between the anode '26 and a grid ,32 to afford a screening action,v

whereby greater output .of the limiter is FigQZ-illustrates the control grid'potentialanode current characteristic of, the limiter 25,

grid potential being .plotted as abscissae and anode currents as ordinates. It willbeobserved that A and B :arevoltages of the control grid24 which produce equal output current of the limiter 25. If thecathode-anode voltage of the 1 device resistor section 23) is so adjusted that B volts-.

are "applied to the control grid 24 when the cathode-anode current of the'udevice l8is cut off and, with zero signal at the. detector l2, the voltvl8(the potential of the age of the cathode of the device l8 is soadjusted that C volts arezapplied to the limiter control 40,

grid 24, the detector input-limiter output char- Otherwise This -1 condition during the -overload or vre- It should be understood (1) that this reduc tion in the output level or the limiter 25 in re- 5o sponse to excessive input level is produced by the i ciently negativeto interrupt the output current such excessive level also drives the grid 11 sum tor l2 should be maintained by AVC or other suitable means at the value A in the center portion of the positive slope of the curve of Fig. 2.

The modulating impulsesthen move up and down.

this portion of the curve and excessive input pro' duces operation in the D region'oi. the curve, resulting in a limiter output current equal to that for zero modulation. I

Itis desirable, although not essential, thatthe radio frequency channel ahead of the detector "l2 be relatively broadly tuned. Following the limiter 25, of course, a normal .audio amplifier may be provided.

I claim as my invention: 1.. In an interference reduction system wherein modulated carrier impulses are applied to a channel including a detector coupled through an amplifier to the control grid of a limiter provided with a cathode, a cathode lead resistor,

an anode and a screen grid, the method of operationwhich comprises reducing the output of said limiter to a lever corresponding to normal zero modulation level during. periods of excessive input, interrupting the output of said amplifier, and maintaining said limiter output at said level.

- 2. In an interferencereduction system wherein modulated carrier impulses are applied to a channel including a detector coupled through an amplifier to the control grid of a limiter provided With a cathode, a cathode lead resistor, an anode and .a screen grid, the method of operation which comprises applying a relativelylow potential to .said anode, applying a relatively high potential 5.5

to said screen grid, interrupting the output of said amplifier during periods of excessive signal 'level,;and'applying to said control grid during .said .periods a potential to maintain the output .ofv said limiter atits normal zero modulation value. 3. The combination of a channel including a detector, an amplifierJpro-videdwith input and output circuits and a limiter provided with in- 'put, control grid and screen grid circuits and having a normal zero modulation output current,

means for. applying a modulated carrier wave .to said detector, means for interrupting the current of said amplifieroutput circuit when the outputof said detector exceeds a predetermined value, and means for maintaining the current of said limiter output circuit at its normal zero modulation value during said interruption.

4. The combination of a channel including a detector, an amplifier provided with input and output circuits and a limiter provided with input, output, controlgrid and screen grid circuits and having a normal zero modulation output current, means for applying a modulated carrier wave to said detector, means for interrupting the current of said amplifier output circuit when the output of said detector exceeds a predetermined value, and means including a potential source common to said amplifier output and said limiter control grid circuits for maintaining the current of said limiter output circuit at its normal zero modulation value during said interruption.

5. The combination of a channel including a detector, anuamplifier provided with input and output circuits and a limiter provided with input,

output, control grid and screen grid circuits and having a normal zero modulation output current,

means for applying a modulated carrier wave to said detector, means for interrupting the current of said amplifier output circuit when the output of. said detector exceeds a predetermined value, andmeans including a resistor common to said amplifier output and said limiter control grid circuits for maintaining the current of said limiter output circuit at its normal zero modulation value during said interruption.

. 6.The combination of a detector, means for applying a modulated carrier to said detector, a limiter including input, output and screen grid circuits and having a normal zero modulation output current, means for applying the output of said detector to said input circuit, and means for reducing the current of said output circuit to its normal zero modulation value when the modulation of said carrier exceeds a predetermined value.

7. The combination of a detector, means for applyinga modulate-d carrier to said detector, a limiter including input, output and screen grid circuits and having a normal zero: modulation output current, means for applying the output, 40

of said detector to said input circuit, means for applying arelatively low potential to said output circuit and a relatively high potential to said screen grid circuit, and means for maintaining the current. of said output circuit at its normal zero modulation value while the modulation of -said carrier exceeds a predetermined value.

8. An interference reduction system comprising in combination a modulated carrier wave signal channel including a detector, an audio,.5o

:frequency amplifier tube having an input circuit coupled to said detector to receive the audio frequency. signal output therefrom and having an output circuit, a limiter tube having a control grid coupled to the output circuit of said audio, frequency amplifier and having an output anode and an output circuit connected therewith, means for applying a modulated carrier Wave to said detector, means for interrupting the output of said amplifier when the output of said detector exceeds a predetermined value, and means for maintaining the anode current of said limiter tubelat a value corresponding to substantially zeromodulation during said interruption.

VERNON D. LANDON. 

